Serum cholesterol binding reserve (SCBR) denotes the amount of cholesterol which a serum sample can solubilize in addition to its cholesterol content. In this study, SCBR of male patients with maturity-onset diabetes, in the age range of 35-59 years, who were treated with insulin, were measured and compared with values of non-diabetic controls of the same sex and in the same age range. The results showed significantly lower SCBR among patients (p less than 0.001). When each patient was matched with a control having serum cholesterol and triglycerides within 5% of the patient's, in 15 of 20 patient-control pairs, the patient had lower SCBR (paired t-test, p less than 0.002). It is suggested that the decreased SCBR in the diabetic patients may be associated with their accelerated atherosclerosis and higher incidence of coronary heart disease. Attempts will be made to identify the defects in the serum lipoproteins which are responsible for decreased SCBR in diabetics.